1. Field of Invention
This invention relates to range gated moving target indicators (RG-MTI), and more particularly to an RG-MTI with expanded range gates.
2. Description of the Prior Art
As known in the art, moving target indicators are used to detect moving targets against a clutter background of stationary targets within a region scanned by the antenna, by utilizing the known Doppler frequency technique. In addition, range gated moving target indicators (RG-MTI) are characterized by the ability to provide excellent range resolution of the moving targets, by quantizing the portion of the pulse repetition period during within which the radar target return signals are received, into range intervals or bins. The width of each range bin is equal, and defines the time interval within which the range gated filters can sample the radar return video signals. In a conventional RG-MTI the time period of the signal frequency of a highly stable clock oscillator, which controls the transmitter pulse repetition frequency, is used to establish the range time interval. Since the target return signals have a pulse width which is equal to or greater than the pulse width of the transmitted radar pulse, the range bin widths are selected to be less than the transmitted pulse width, to prevent the loss of a target return signal due to "straddling" of the target return signal by a range bin having a time interval larger than the return signal pulse width. Therefore, the pulse repetition period must be quantized into a number of such range bins, all having a time interval less than the pulse width of the main bang signal, with the total number of required range gated filters being equal to the system range coverage divided by the transmitted pulse width. Since each range gated filter circuit is comprised of an input gate, a zero order data hold or box car circuit, a Doppler filter (either analog or digital), and an output gate, the cost of providing the required number of range bin circuits for extended range radar systems becomes appreciable.
As a result of the required narrow range bin widths, the sensed range resolution of the moving target is excellent, however, due to the inability to display the detected target at a resolution comparable to that of the detected range, the overall system resolution is reduced and limited by the displayable resolution. To illustrate, in a typical radar where a 5 mile coverage is displayed, using 200 nanoseconds range bin widths, and a 200 nanosecond transmitted pulse width, the sensed range resolution is approximately 100 feet out of 30,000 feet or 0.3 percent. If the 5 mile coverage is displayed on a typical five inch PPI display, the 0.3 percent resolution represents a display increment of 0.008 inch which, as may be appreciated, is almost imperceptible to the viewer. Typical prior art RG-MTI's correct this invisibility problem by "smearing" the moving target video in the output circuitry of the RG-MTI so that an appropriate size indication appears on the display. This "smearing" in effect, decreases the displayed range resolution of the RG-MTI to generally one to two percent. This reduction in range resolution by "smearing" thereby discards the higher range resolution provided by the narrow range bins, which of necessity must be of a time interval equal to or less than the transmitted pulse width. Thus, it would be preferred to have a system which provides the reduced range resolution required for visibility of displayed target signals by increasing the range bin width, thereby reducing the number of range gated filters, while still preserving the ability to detect target return signals which are smaller in duration than the range bin (prevent "staddle" loss). All of these features may be accomplished in a system which increases the pulse width of the incoming radar video signals by a factor which is equal to the increased range bin width.
One circuit for stretching the radar video return signals in an RG-MTI is described in a patent to Castets et al, U.S. Pat. No. 3,713,152, wherein the radar return signals, after processing in a phase detector which extracts the Doppler frequency component of return video signals, is applied to a number of serially connected delay lines, the output of which are summed and presented to the range gated filter inputs. The number of the delay lines and the delay time constant are selected in dependence on the degree of disparity between the range bin width and the pulse width of the transmitted radar signal. This system suffers from the disadvantage that the series delay lines cause attenuation of the incoming signal and therefore the re-amplification of the signal is necessary. In addition, there is a requirement for using isolators at the input and output of each delay line if the delay lines are not well matched. A further disadvantage appears to reside in the inherent time lag of the low pass filter which couples the stretched video signal from the summing circuit to the range gated filters, which may cause spill over of the video signal into adjacent range bins which adversely affects the range resolution of the system.